The long-term goal of the principal investigator's studies is to elucidate the mechanisms of action of the proinflammatory cytokines, tumor necrosis factor-alpha (TNF) and interleukin-1 (IL-1). The major aim of the present application is to elucidate the role of the TNF/IL-1-inducible TSG-6 protein in inflammation, especially in inflammatory and degenerative joint diseases. Earlier studies from the principal investigator's laboratory demonstrated that high levels of TSG-6 protein are present in the synovial fluids of patients with rheumatoid arthritis and some other forms of arthritis. The TSG-6 protein was shown to have the following activities: (a) TSG-6 binds to hyaluronan, a major component of the extracellular matrix, (b) TSG-6 forms a stable complex with components of the serine protease inhibitor, inter-alpha-inhibitor (I-alpha-I), (c) TSG-6 potentiates the plasmin-inhibitory activity of I-alpha-I, and (d) recombinant TSG-6 protein exerts a potent antiinflammatory action in a mouse model. The first specific aim is to clarify the mechanism of the antiinflammatory action of TSG-6. These studies should reveal whether the antiinflammatory action of TSG-6 is causally related to any of its other demonstrated activities, thereby either confirming or refuting the principal investigator's hypothesis that inhibition of plasmin activity via interaction with I-alpha-I is central to the antiinflammatory action of TSG-6. Cell culture models of inflammation will be devised in which the effects of wild-type and mutant TSG-6 proteins can be evaluated. The second major aim is to determine the role of endogenously produced TSG-6 protein in the intact organism. It will be determined whether targeted deletion of the tsg6 gene or overexpression of human TSG-6 protein in transgenic mice leads to phenotypic alterations and whether it affects susceptibility toward the induction of acute or chronic (collagen-induced arthritis) inflammatory responses. Transgenic mice and tsg6 knockout mice will also be used to determine if TSG-6 plays a role in other disease models involving TNF or IL-1 actions: the lethal action of bacterial lipopolysaccharide and susceptibility to intracellular bacterial infection.